1. Field of the Invention
The present invention generally relates to electro-optical imaging systems using solid-state sensors for recording a target image, including optical code readers and digital cameras. Aspects of the invention are particularly useful in linear sensor-based and two-dimensional sensor-based, handheld bar code readers. More specifically, the present invention relates to simulating the appearance of a flickering laser beam in such imaging systems.
2. Description of the Related Art
Optical codes are patterns made up of image areas having different light reflective or light absorptive properties, which are typically assembled in accordance with a priori rules. The term “bar code” is sometimes used to describe certain kinds of optical codes. The optical properties and patterns of optical codes are selected to distinguish them in appearance from the background. Devices for identifying or extracting data from optical codes are sometimes referred to as “optical code readers” of which bar code scanners are one type. Optical code readers are used in both fixed and portable installations in many diverse environments such as in stores for checkout services, in manufacturing locations for work flow and inventory control, and in transport vehicles for tracking package handling. The optical code can be used as a rapid, generalized means of data entry, for example, by reading a target bar code from a printed listing of many bar codes. In some uses, the optical code reader is connected to a portable data processing device or a data collection and transmission device. Frequently, the optical code reader includes a handheld sensor which is manually directed at a target code.
Most conventional code readers are designed to read one-dimensional bar code symbols. The bar code is a pattern of variable-width rectangular bars separated by fixed or variable width spaces. The bars and spaces have different light reflecting characteristics. One example of a one-dimensional bar code is the UPC/EAN code.
Bar codes have traditionally been read by sweeping a laser beam across the codes, and by detecting light reflected off the codes. The detected light is electronically processed to generate data related to the code. The laser beam is swept at a scanning rate of many times per second, e.g., 20 or 40 scans per second. To a user, the laser beam appears to flicker and, hence, over the course of time, a user expects to see a flickering laser beam during reading of the codes. The flickering beam serves as a visual cue that the system is in operation.
Bar codes can also be read by employing solid-state imaging devices. For example, an image sensor may be employed which has a two-dimensional array of cells or photosensors which correspond to image elements or pixels in a field of view of the device Such an image sensor may be a two-dimensional or area charge coupled device (CCD) and associated circuits for producing electronic signals corresponding to a two-dimensional array of pixel information for a field of view. A one-dimensional linear array of photodiodes may also be used in detecting a bar code reflection image (see, e.g., U.S. Pat. No. 6,138,915 to Danielson et al., which is herein expressly incorporated by reference).
It is known in the art to use a CCD image sensor and objective lens assembly in an optical code reader. In the past, such systems have employed complex objective lens assemblies originally designed for relatively expensive video imaging systems. Such systems may have a single sharp focus and a limited depth of field, which along with conventional aiming, illumination and signal processing and decoding algorithms, limits the versatility and working range of the system.
However, the solid-state imagers provide no visual cue as to their operation. There is no flickering laser beam. A user, expecting a flickering beam, instead sees nothing since the solid-state imager acts like a camera and merely receives light from the target.
To provide illumination and to assist in aiming, imaging systems can employ either lasers or light emitting diodes (LEDs). LEDs may be preferred over lasers since the incoherent nature of the LED light source does not produce the speckle noise impact that is produced by lasers. Further, LEDs are more cost effective than lasers due to the ease of manufacturing and packaging of LEDs. Additionally, LEDs can be built more compactly and are easier to surface mount than lasers.
However, compared to lasers, LEDs are not an ideal point source. Specifically, light produced by an LED is less focused which produces an increased line thickness of the projected light. To reduce the line thickness of the light produced by an LED, many designers place a mechanical slit in front of the LED. However, the mechanical slit reduces the amount of light that is projected by the LED onto an object. In any event, during use, the LED is constantly energized to maximize the collected light and to enhance the aiming.